Anomalous transport effects in the impedance of porous film electrodes

The behavior of the ac conductivity in disordered materials where the charg e carriers (electrons, holes, polarons, ions) proceed by discrete transitio ns between localized states exhibits a universal pattern consisting of a po wer-law domain at high frequencies and a constant conductivity in the low-f requency wing. Such behavior is likely to occur in several types of new ele ctrode materials such as nanostructured semiconductors and electroactive po lymers. We suggest new models for the analysis of these systems by impedanc e. The standard double-channel transmission line for the impedance of porou s electrodes is endowed with a description of anomalous transport effects. The modeling of transport in a disordered medium is based on a macroscopic phenomenological theory that accounts for the retarded nature of the respon se to a locally acting force due to the localization of the carriers. The i nfluence of the power-law behavior in the solid matrix gives rise to a bend ing of the 'Warburg' part of the impedance spectra, or to an are at high fr equency, among other possible responses. (C) 1999 Elsevier Science S.A. All rights reserved.